The present invention relates to an extrusion method for forming plastic film. In particular, this invention relates to a process, and the products formed thereof, for forming a lenticular or non-lenticular optical product without the need of a carrier sheet that involves extrusion of at least one layer of plastic material, at least a three-roller system, and a continuous belt.
A lenticular plastic is a sheet of clear plastic that has a series of lenses running along its length. Alternately, a non-lenticular pattern may be used to direct rays of light to appropriate positions on or behind the sheet. In general, processes for making extruded lenticular-coated substrates are known. However, these processes suffer the disadvantage of requiring a high-cost substrate or carrier sheet material, which may represent one-half to two-thirds of the cost of the lenticular material. Although the carrier sheet material is relatively expensive, it has been necessary to provide it because it is essential for proper optical quality.
Lenticular-coated substrates can also be made on sheet lines or the lenticular layer can be cast using thermoplastic resins. However a sheet line process leads to unrestrained shrinkage during the cooling cycle and, thus, deformation of the lenses. Casting of thermoplastic resins is undesirable because this process is relatively slow and employs expensive resins.
This invention is based on the discovery that such optical distortion can be reduced, and the production of high-quality extruded optical sheets (lenticular and non-lenticular) without a carrier sheet (substrate) can be facilitated by controlling the shrinkage of one side of the extruded material. It has been found that the necessary control can be achieved in a three-roll system with the use of a continuous belt. The elimination of the carrier sheet produces a savings of about $1.50 to about $3.00 a pound as of the date of filing of this application, which may be more than half of the cost of the optical sheet. The reduction in cost should make possible more widespread application of this material in such uses as photographic material, flexible packaging, promotional materials, T-shirts, and other uses where printed matter can be used.